THE WONDERS OF LIFE 



species of fungi cause certain diseases by their poison- 

 ous (chemical) action on the tissues of their host. It is 

 well known how our most important cultivated plants, 

 the vine, potato, corn, coffee, etc., are threatened by 

 fungoid diseases; and this is also true of many of the 

 lower and higher animals. It is probable that the fungi 

 have been evolved polyphyletically by metasitism from 

 the algas. 



Among the higher metaphyta we find parasitism in 

 many different families, especially orchids, rhinantha- 

 cea {orobranche, lathraca), convolvulacea {citsciUa), aris- 

 tolochiacea, loranthacea (visctim, loranthus), rafffesiacea, 

 etc. These various kinds of flowering-plants often grow 

 to resemble each other by convergence (that is to say, 

 by their common adaptation to parasitic life) ; they 

 lose their green leaves, the plasmodomous chlorophyll 

 of which is of no further use to them. Frequently rudi- 

 mentary leaves are left on them in the form of colorless 

 scales. For the purpose of clinging to the plants they 

 live on, and penetrating into their tissues, they evolve 

 special clinging apparatus (haustoria, suctorial cups, 

 creepers). Their stalks and roots are also modified in a 

 characteristic way. The whole productive force of these 

 parasites is expended on their sexual organs; rafflesia 

 has the largest flowers there are, more than a yard in 

 diameter. 



Parasitism in the metazoa (in all groups) is even more 

 frequent and interesting than in the metaphyta. The 

 mollusks and echinoderms show the least disposition for 

 it, and the platodes, vermalia, and articulates the most. 

 Even among the gastrasada, the common ancestral group 

 of the metaphyta, we find parasites (kyemaria and gas- 

 tremaria). The protection they find inside their hosts 

 is probably the reason why these oldest of the metazoa 

 have remained unchanged to the present day. Real 

 parasites are not numerous among the sponges and 



236 / 



